Test Considerations - Agilent Technologies 3458A Calibration Manual

Test Considerations

1. Normal mode noise rejection is the multimeter's ability to reject noise at the power line frequency from DC
voltage, DC current, or resistance measurements.
34 Chapter 4 Performance Verification Tests
This section discusses many of the major problems associated with low-level
measurements. Many of the measurements in this manual fall into this
category. It is beyond the scope of this manual to go into great detail on this
subject. For more information, refer to a textbook dealing with standard
metrology practices.
•
Test leads: Using the proper test leads is critical for low-level
measurements. We recommend using PTFE cable or other high
impedance, low dielectric absorption cable for all measurement
•
Connections. It is important to periodically clean all connection points
(including the multimeter terminals) using a cotton swab dipped in
alcohol.
•
Noise Rejection: For DC voltage, DC current, and resistance
measurements, the multimeter achieves normal mode noise rejection
1
(NMR) for noise at the A/D converter's reference frequency (typically
the same as the power line frequency) when the integration time is ³ 1
power line cycles. You can specify integration time in terms of power
line cycles (PLCs) using the NPLC command. For maximum NMR of
80dB, set the power line cycles to 1000 (NPLC 1000 command).
•
Guarding: Whenever possible, make measurements with the
multimeter's Guard terminal connected to the low side of the
measurement source and the Guard switch set to the Open position
(guarded measurements). This provides the maximum effective
common mode rejection (ECMR).
•
Thermoelectric Voltages (Thermal EMF): This is a common source of
errors in low-level measurements. Thermal EMF occurs when
conductors of dissimilar metals are connected together or when different
parts of the circuit being measured are at different temperatures.
Thermal EMF can become severe in high-temperature environments.
To minimize thermal EMF, minimize the number of connections: use
the same type of metal for all connections; minimize the temperature
variations across the measurement wiring: try to keep the multimeter
and the wiring at the same temperature: and avoid high-temperature
environments whenever possible.
•
Electromagnetic Interference (EMI): This type of interference is
generally caused by magnetic fields or high levels of radio frequency
(RF) energy. Magnetic fields can surround all types of equipment
operating off of AC line power, especially electric motors. RF energy
from nearby radio or television stations or communications equipment
can also be a problem. Use shielded wiring whenever the measurement
setup is in the presence of high EMI. If possible, move farther away or